Did you know one dollar bills are made in twelve different cities? They are made in Boston, New York City, Philadelphia, Cleveland, Richmond, Atlanta, Chicago, St. Louis, Minneapolis, Kansas City, Dallas, and San Francisco. Each has a letter associated with the city. They are, in order, A Boston, B New York, C Philadelphia, etc. Those letters are all associated with the numbers printed four times on the bill. The black letter in the seal on the left side of the bill tells us which city it is printed in.
The four black numbers toward each corner correspond to the letter. So, a bill printed in Philadelphia has a C in the seal and would have the number three printed, because C is the third letter of the alphabet. Incidentally, they print 16,600,000 bills a day. I just checked my pocket and had bills printed in four different cities.